1. Technical Field
The present invention relates to a discharge lamp lighting circuit to maintain the lighting state thereof and to restart at the time of the extinction or turning-off thereof.
2. Related Art
As a lighting circuit for a discharge lamp such as a metal halide lamp used for an illumination light source for a vehicle, there is the configuration which includes a DC power source circuit having a DC-DC converter, a DC/AC converter and a starting circuit. For example, this configuration is arranged in a manner that a DC input voltage from a battery is converted into a desired voltage by the DC power source, then the desired voltage is converted into an AC output by the DC/AC converter of the succeeding stage, and a start signal is superimposed on the AC output and supplied to the discharge lamp (see Japanese Patent document JP-A-7-142182, for example).
The lighting control of the discharge lamp is performed in a manner that an output voltage at the time of no-load or an open circuit voltage (hereinafter referred to OCV) in a state (extinction state) before lighting the discharge lamp is controlled to apply the start signal to the discharge lamp thereby to light or turn on the discharge lamp, and thereafter the discharge lamp is shifted to a steady lighting state while reducing a transient input power.
A switching regulator using a transformer, for example, is employed as the DC power source circuit. A full-bridge configuration using plural pairs of switching elements, for example, is employed as the DC/AC converter.
According to the configuration of performing the two-stage conversion of the DC voltage conversion and the DC/AC conversion, since the size of the circuit becomes large, this configuration is not suitable for miniaturization. As a measure to obviate such a problem, there is the configuration which employs the one-stage voltage conversion of the DC/AC converter thereby to supply a boosted voltage to the discharge lamp.
For example, there is the configuration which includes a series resonance circuit and is arranged to boost a resonance voltage by a transformer and electric power is supplied to the discharge lamp based on the boosted voltage. The series resonance circuit constituted by a capacitor and an inductance element has frequency characteristics almost symmetrical with respect to the resonance frequency and an output voltage, and an electric power can be controlled by changing the driving frequency of a semiconductor switching element constituting the DC/AC converter. The series resonance circuit has a tendency that the output voltage reduces in accordance with the increase of the frequency in the frequency range (an inductive area or a phase delay area) higher than the resonance frequency, whilst the output voltage reduces in accordance with the reduce of the frequency in the frequency range (a capacitive area or a phase advance area) lower than the resonance frequency.
In the OCV control in the extinction state before the lighting after turning-on of the power source, when the driving frequency of the semiconductor switching element is reduced in the frequency range higher than a series resonance frequency “Foff”, the OCV value is increased. When the OCV value reaches a target value, a starting high voltage pulse is generated and applied to the discharge lamp. When the discharge lamp lights or turns on, the driving frequency is shifted to a frequency range higher than a series resonance frequency “Fon” (higher than Foff) thereby to start the electric power control of the discharge lamp.
In the control relating to the driving frequency of the semiconductor switching element on the condition that the output is controlled on the higher frequency side than the resonance frequencies (Foff and Fon) at the time of the extinction and lighting of the discharge lamp, respectively, there arises an abuse caused by a fact that the driving frequency of the semiconductor switching element is one-sidedly controlled so as to be reduced by the reduction of the power source voltage or the grounding at the output stage etc.
That is, in the frequency region (the inductive area) higher than the resonance frequency, the driving frequency is controlled so as to be reduced in order to increase the output voltage and the power, whilst the driving frequency is controlled so as to be increased in order to reduce the output voltage and the power. Thus, the aforesaid control acts on the output voltage and the power undesirably when a control operation point enters into the frequency region (the capacitive area) lower than the resonance frequency due to a some reason (for example, when the input voltage from a battery etc. reduces or when it is determined that the discharge lamp is extinguished by the grounding). That is, when the driving frequency is reduced in order to increase the output, the output reduces accordingly, so that the driving frequency is controlled so as to be further reduced. This is because the output voltage tends to reduce with respect to the reduction of the frequency in the frequency region lower than the resonance frequency, whereby the driving frequency of the semiconductor switching element reduces endlessly and it becomes impossible to escape from such a state. As a result, there may arise such a problem that the discharge lamp can not be lighted again or the controlling of a target power becomes difficult. Thus, it is required to provide a measure for eliminating such a problem.